New Audio Test Gear 2008

The 2008 initiative to improve SPCR's audio testing process involved the construction of an anechoic chamber and upgrades of test equipment to take advantage of the lower noise floor in the chamber. The chamber development is detailed in An Anechoic Chamber for SPCR; this article covers the new test equipment details.

The essence of the challenge was to acquire audio test equipment capable of making full use of the super-low noise floor of the new anechoic chamber. The accuracy of our existing gear was limited to perhaps 18~20 dBA, below which the results could not be trusted. The new anechoic chamber could bring the noise floor down to 10 dBA, so I was seeking audio test gear capable of measuring down that low accurately.

The choice from the start was...

A. Get a new microphone for better recordings and a new SLM to measure lower levels. Discrete tools mean no interdependence; if one fails, the other keeps working. A portable modern digital SLM would be very convenient, as they are small yet extremely powerful. The main problem: Cost. The cheapest microphone suitable for our purpose would be $2,000 and it would take $10,000 to obtain a SLM capable of <15 dBA readings.

B. The alternative was to combine the two functions: A new microphone with a computer-based SLM. A PC-based audio spectrum analyzer can make use of the incredible power of today's computers, and the data input microphone could be the same one used for recording. The cost would naturally be much lower than option A. This was the obvious choice.

EXISTING TEST EQUIPMENT

A Brüel & Kjær model 2203 analog sound level meter made in the 1970s had been the core of SPCR SPL measurements for years. Its absolute accuracy is probably questionable, but it has the capability to read lower levels than most modern digital SLMs priced under $10,000. The meter's range is just 30 dB, but switchable from -10 dB to 140 dB. The main limitation is that the internal noise of the 1" microphone and the meter electronics is about 16 dBA. Any lower than that, the readings are highly questionable. It's likely that we're hitting the limits of this SLM when checking ambient noise levels in the house after midnight. An upgrade seemed unavoidable.

Good down to 16~18 dBA when new.

A Sennheiser ME 66 had been our recording microphone for several years. It is rated for an equivalent noise level of 10 dBA or CCIR-weighted 21 dB, which is very quiet. It also has a very high output level, which is useful in keeping the noise down. The 10 dBA self-noise claim may be exaggerated, however. Research into microphone noise and discussions with technical reps from a variety of microphone manufacturers suggested that such a low self-noise is not achievable with a microphone relying on +48V phantom power.

The ME 66 has a highly directional ("super-cardioid") pickup pattern that intentionally isolates the subject from the background noise. It is intended for use in the film and broadcasting industries, where it is popular among independent and documentary filmmakers. Its frequency response is not perfectly flat, as the visual below shows. The 7~12 kHz peak is audible; it helps with voice intelligibility. The dropoff below 400 Hz is also audible; it sounds thinner compared to another omnidirectional, more linear mic we have on hand.

The bass rolloff and high frequency peaks are both audible, so the ME 66 is history.

I came to the conclusion after several weeks of research that the Sennheiser ME 66 is not adequately quiet or linear enough to stay with us to the next step.

An M-Audio Tampa professional microphone/instrument preamp with integrated 24-bit / 96-kHz A/D converter is the primary hardware interface between the microphone and the computer software used for recordings and signal analysis. Now discontinued, the specs remain excellent: Noise is rated at 110 dBA. Although its digital signal is fed through an M-Audio FireWire 410, the Tampa is responsible for both signal gain as well as analog-to-digital conversion; The FW410 doesn't actually do anything to the digital signal. The FireWire 410's main function here is for playback and monitoring via headphones. Both M-Audio gear are good enough to continue using for the time being.

At this point, there was a choice:

A. Get a new microphone for better recordings and a new SLM to measure lower levels. Discrete tools mean no interdependence; if one fails, the other keeps working. A portable modern digital SLM would be very convenient, as they are small yet extremely powerful. The main problem: Cost. The cheapest microphone suitable for our purpose would be $2,000 and it would take $10,000 to obtain a SLM capable of 15 dBA readings.

B. The alternative was to combine the two functions: A new microphone with a computer-based SLM. A PC-based audio spectrum analyzer can make use of the incredible power of today's computers, and the data input microphone could be the same one used for recording. The cost would naturally be much lower than option A. This was the obvious choice.